This invention relates to electrochemical cell batteries, particularly to cells with increased interfacial surface area between the positive and negative electrodes.
Batteries containing electrochemical cells are used as power sources for electrical devices. An ideal battery would be one that is inexpensive, with unlimited capacity regardless of power level, temperature or operating conditions. It would also have an unlimited storage life, be safe under all conditions, and be impossible for the user to misuse or abuse. While such an ideal battery is not possible, battery manufacturers continue to design batteries that will come closer to that ideal. In a practical battery, there are tradeoffs and compromises that must be made among the ideal battery characteristics related to battery performance. Thus, the requirements of the electrical devices that will be powered by the battery are important factors in battery and cell design. For example, many devices have battery compartments that limit the size and shape of the battery or batteries, and the discharge characteristics of the battery/batteries must be sufficient to operate the device under expected conditions of use.
Manufacturers are continually trying to increase the capabilities and the number of features of electrical devices. This results in ever-increasing demands for batteries that will provide higher power without unacceptable sacrifices in the other desirable battery performance characteristics, such as long discharge life (high capacity), long storage life, resistance to leakage, and ease of manufacture. This trend in increasing power requirements is evident in portable devices with consumer-replaceable batteries.
Achieving high battery capacity and long discharge life is especially challenging at high discharge rates required for high power because batteries are able to deliver only a fraction of their theoretical capacity, and that fraction (the discharge efficiency) decreases as the discharge rate increases. There are many factors that contribute to the discharge efficiency of batteries and the cells they contain. One factor is the interfacial surface area between the electrodes. Increasing the interfacial surface area generally has positive effects on current density, internal resistance, concentration polarization, and other characteristics that can effect discharge efficiency. However, increasing the interfacial surface area often comes at the expense of reduced active materials and theoretical discharge capacity. In designing a cell with increased interfacial surface area it is desirable to minimize necessary reductions in active materials, increases in inert components, and increases in expensive materials that do not themselves improve performance, as well as any other changes that reduce the theoretical capacity or otherwise offset improvements.
Some consumer batteries use active materials and/or electrolytes that are especially well suited for high power applications. Examples include primary lithium batteries and rechargeable (secondary) nickel/cadmium batteries. These batteries often use materials that are relatively expensive, have special handling requirements, or raise environmental concerns in the disposal of spent batteries. Because high interfacial surface area is generally preferred for high rate/high power applications, these batteries often have spiral wound electrode designs. However, these designs usually have more internal volume consumed by separators and current collectors and are generally more difficult and expensive to manufacture than bobbin designs.
The use of alkaline zinc/manganese dioxide batteries can solve these problems if the device does not exceed their power requirements. There is a need to improve the high power capability of alkaline batteries to make them suitable as power sources for higher power devices.
In a cylindrical alkaline Zn/MnO2 cell with a bobbin-type construction, high rate discharge performance can be improved by increasing the electrode interfacial surface area. Typical commercial cells of this type have a positive electrode disposed next to the can. This positive electrode (cathode) has essentially a hollow cylindrical shape with a smooth, round internal surface, within which the separator and negative electrode (anode) are disposed. The electrode interfacial surface area can be increased by changing the internal surface of the positive electrode so that it is no longer smooth. One convenient way to do this, which is compatible with typical cell manufacturing processes in use, is to corrugate the positive electrode surface, with the corrugations running vertically (i.e., parallel to the can side walls when the positive electrode is assembled into the can). In general, the higher the surface area, the better the high rate discharge capacity. Additional improvement in high rate discharge capacity may also be realized if the cathode thickness is generally reduced, since this will tend to reduce polarization of the positive electrode.
There have been previous attempts to improve the high power capability of alkaline batteries by increasing electrode interfacial area. Examples can be found in U.S. Pat. No. 5,869,205, No. 6,074,781 and No. 6,342,317. However, each of these references suffers from one or more of the following disadvantages.
Manufacture of cells is difficult when a current collector prong must extend into each of a plurality of like-polarity electrodes. This means that each current collector prong must be aligned with one of the plurality of electrodes, requiring orientation of both the cell and the current collector. In addition, when multiple current collector prongs are required, the volume of active materials must be reduced to allow for an increase in the total volume of the collector, compared to cell designs in which a single current collector prong will suffice.
The use of typical separator materials (e.g., polymeric film and woven or nonwoven paper or fabric) in strip or sheet form may be impractical due to difficulty in making the separator conform to the surface of the cavity in the cathode. Even application of a spray-on separator to the interfacial surface of one of the electrodes can be difficult. Sharp corners and non-vertical interfacial surfaces can also make it difficult to completely fill the cavity with anode at the high speeds desirable in manufacturing.
When discharge efficiency is maximized by making the maximum distance of active material in a first electrode from an interfacial surface of a second electrode, the resulting first electrode shape can create various problems during cell manufacture: (1) difficulty in inserting the separator so that the entire interfacial surface of the first electrode is covered by separator without leaving voids between the electrode and the separator, (2) difficulty in keeping the separator against the first electrode surface so gaps do not develop before, during or after insertion of the second electrode, and (3) preventing the formation of air pockets between the second electrode and separator during high speed cell assembly. Such electrode shapes also tend to include relatively fragile lobes or projections extending from the electrode surfaces, making breakage more likely during electrode forming and handling, as well as during and after assembly of the electrodes and separator into the cell container.
The smaller diameter cells (e.g., AA/R6 and AAA/R03 sizes) are particularly susceptible to the above problems due to more the more limited spaces available and the need to make the electrode dimensions smaller.
In view of the above principles, an object of the present invention is to provide an electrochemical battery cell that is inexpensive and easy to manufacture, has high capacity, performs well under expected temperature and operating conditions, has long storage life, is safe, and is not prone to failure as a result of misuse or abuse by the user.
Another object of the present invention is to provide a battery cell that has improved high rate/high power discharge performance with minimal adverse effects on theoretical capacity, discharge performance at moderate and low rates, and other desirable battery cell characteristics.
It is also an object of the present invention to provide an economical battery cell with electrodes having a high interfacial surface area.
In view of the above problems with cell designs having high electrode interfacial surface area, it is a further object of the present invention to provide an economical, reliable alkaline zinc/manganese dioxide battery cell with a bobbin-type electrode configuration, capable of high speed mass production, that has a high electrode interfacial surface area.